Objectives: 1. Define the role for the pituitary in leukemogenesis. Hypophysectomy will be performed in rats at varying intervals beginning at 3 weeks of age. These animals will then be assessed for their susceptibility to leukemia induced in 3 ways, Gross murine leukemia virus, 7,12 dimethylbenzanthracene, and x-irradiation. 2. Identify the hormone or unknown pituitary factor necessary for induction of leukemia in the experimental model. Pituitary extract or purified pituitary hormones will be injected into hypophysectomized rats in an effort to restore the normal process of leukemogenesis. 3. Delineate the thymic-pituitary interaction in the induction process. Study the effect thymectomized rats of thymosin with or without the pituitary hormones on the induction of a T-cell leukemia.